1. Field of the Invention
This invention relates to a magnetic component for an electric machine, such as a motor; and more particularly, to a method of constructing a low core loss, unitary amorphous metal component, such as a rotor or stator, for a high efficiency, axial-flux electric motor.
2. Description of the Prior Art
Rotating electric machines almost always comprise at least two magnetic components, a stationary component termed a stator and a rotor appointed to rotate relative to the stator and about a defined rotation axis. Such a rotating machine allows energy to be exchanged between electrical and mechanical forms. Most familiarly, an electric motor is provided with a source of electrical energy, as from a battery or the electric power grid, that may be converted to usable mechanical work. On the other hand, a generator takes imposed mechanical work and converts it to electrical energy that may be used to operate other devices. In many cases the same structure may be used for both functions, depending on how the machine is connected electrically and mechanically.
A vast majority of rotating electrical machines operate electromagnetically. In such machines the rotor and stator normally comprise ferromagnetic materials. The components are used to either produce or direct a pattern of magnetic flux that varies either temporally or spatially or both. The conversion of energy between electrical and mechanical forms occurs in accordance with the well known principles of electromagnetism, especially Faraday's and Ampère's laws. In electromagnetic machines at least one of the rotor and stator is constructed using a soft ferromagnetic material and provided with a winding appointed to carry an electrical current and generate a magnetic field. Depending on the type of motor, the other component includes either permanent (hard) magnetic material or soft magnetic material that is excited by current-carrying windings or by induction. The soft magnetic materials most commonly used are low carbon steels and silicon-containing electrical steels, both of which are crystalline metallic materials.
The stator and the rotor in a machine are separated by small gaps that are either (i) radial, i.e., generally perpendicular to the axis of rotation of the rotor, or (ii) axial, i.e., generally parallel to the rotation axis and separated by some distance. In an electromagnetic machine, lines of magnetic flux link the rotor and stator by traversing the gaps. Electromagnetic machines thus may be broadly classified as radial or axial flux designs, respectively. The corresponding terms radial gap and axial gap are also used in the motor art.
Radial flux machines are by far most common. The rotors and stators used in such motors are frequently constructed of a plurality of laminations of electrical steel that are punched or otherwise cut to identical shape, stacked in registry, and laminated to provide a component having a requisite shape and size and sufficient mechanical integrity to maintain the configuration during production and operation of the motor.
One common design for a stator is generally cylindrical and includes a plurality of stacked laminations of non-oriented electrical steel. Each lamination has the annular shape of a circular washer along with plural “teeth” that form the poles of the stator. The teeth protrude from the inner diameter of the stacked laminations and point toward the open center of the cylindrical stator. Each of the laminations is typically formed by stamping mechanically soft, non-oriented electrical steel into the desired shape. The formed laminations are then stacked in registry and bonded to form a stator. During operation, the stator is periodically magnetized by a magnetic field, produced by a flow of electric current in windings that encircle the teeth of the stator. Such magnetization is needed to drive the motor; but causes unavoidable losses due to magnetic hysteresis. These losses contribute to an overall reduction in motor efficiency.
Axial flux designs are much less commonly used, in part because of the lack of suitable means for constructing components having the requisite electromagnetic properties and adequate mechanical integrity. Certain disclosures have suggested axial flux motor designs, including those found in U.S. Pat. No. 4,394,597 to Mas and U.S. Pat. No. 5,731,649 to Caamano. These teachings also suggest magnetic components that employ amorphous metals.
Although amorphous metals offer superior magnetic performance, including reduced hysteresis losses, when compared to non-oriented electrical steels, they have widely been regarded as not suitable for use in electric motors due to certain physical properties and the resulting impediments to conventional fabrication. For example, amorphous metals are thinner and harder than non-oriented steel. Consequently, conventional cutting and stamping processes cause fabrication tools and dies to wear more rapidly. The resulting increase in the tooling and manufacturing costs makes fabricating amorphous metal components such as rotors and stators using conventional techniques commercially impractical. The thinness of amorphous metals also translates into an increase in the number of laminations required for a component of a given stack height, further increasing its total manufacturing cost.
Amorphous metal is typically supplied in a thin continuous ribbon having a uniform ribbon width. However, amorphous metal is a very hard material, making it very difficult to cut or form easily. Once annealed to achieve peak magnetic properties, it becomes very brittle, making it difficult and expensive to use conventional approaches to construct amorphous metal magnetic components. The brittleness of amorphous metal also causes concern for the durability of a motor or generator that utilizes amorphous metal magnetic components. Magnetic stators are subject to extremely high magnetic forces, which change at very high frequencies. These magnetic forces are capable of placing considerable stresses on the stator material, and may damage an amorphous metal magnetic stator. Rotors are further subjected to mechanical forces due both to normal rotation and to rotational acceleration when the machine is energized or de-energized and when the loading changes, perhaps abruptly.
Another problem with amorphous metal magnetic components is that the magnetic permeability of amorphous metal material is reduced when it is subjected to physical stresses. This reduction in permeability may be considerable depending upon the intensity of the stresses on the amorphous metal material, as indicated by U.S. Pat. No. 5,731,649. As an amorphous metal magnetic stator is subjected to stresses, the efficiency at which it directs or focuses magnetic flux is reduced, resulting in higher magnetic losses, reduced efficiency, increased heat production, and reduced power. This phenomenon is referred to as magnetostriction and may be caused by stresses resulting from magnetic forces during the operation of the motor or generator, mechanical stresses resulting from mechanical clamping or otherwise bonding or fixing the magnetic stator in place, or internal stresses caused by the thermal expansion and/or expansion due to magnetic saturation of the amorphous metal material.
A limited number of non-conventional approaches have been proposed for constructing amorphous metal components. For example, U.S. Pat. No. 4,197,146 to Frischmann discloses a stator fabricated from molded and compacted amorphous metal flake. Although this method permits formation of complex stator shapes, the structure contains numerous air gaps between the discrete flake particles of amorphous metal. Such a structure greatly increases the reluctance of the magnetic circuit and thus the electric current required to operate the motor.
In order to avoid stress-induced degradation of magnetic properties, U.S. Pat. No. 5,731,649 discloses constructing amorphous metal motor components using a plurality of stacked or coiled sections of amorphous metal, and mounting these sections in a dielectric enclosure. The '649 patent further discloses that forming amorphous metal cores by rolling amorphous metal into a coil with lamination, using an epoxy, detrimentally restricts the thermal and magnetic saturation expansion of the coil of material, resulting in high internal stresses and magnetostriction that reduces the efficiency of a motor or generator incorporating such a core.
The approach taught by German Patents DE 28 05 435 and DE 28 05 438 divides the stator into wound pieces and pole pieces. A non-magnetic material is inserted into the joints between the wound pieces and pole pieces, increasing the effective gap, and thus increasing the reluctance of the magnetic circuit and the electric current required to operate the motor. The layers of material that comprise the pole pieces are oriented with their planes perpendicular to the planes of the layers in the wound back iron pieces. This configuration further increases the reluctance of the stator, because contiguous layers of the wound pieces and of the pole pieces meet only at points, not along full line segments, at the joints between their respective faces. In addition, this approach teaches that the laminations in the wound pieces are attached to one another by welding. The use of heat intensive processes, such as welding, to attach amorphous metal laminations will recrystallize the amorphous metal at and around the joint. Even small sections of recrystallized amorphous metal will normally increase the magnetic losses in the stator to an unacceptable level.
Moreover, amorphous metals have far lower anisotropy energies than other conventional soft magnetic materials, including common electrical steels. As a result, stress levels that would not have a deleterious effect on the magnetic properties of these conventional metals have a severe impact on magnetic properties important for motor components, e.g. permeability and core loss. For these reasons, U.S. Pat. No. 5,731,649 discloses a magnetic component comprising a plurality of segments of amorphous metal carefully mounted or contained in a dielectric enclosure without the use of adhesive bonding.
Notwithstanding the advances represented by the above disclosures, there remains a need in the art for methods of constructing improved amorphous metal motor components that exhibit a combination of excellent magnetic and physical properties needed for high speed, high efficiency electric machines, especially axial flux designs. Construction methods are also sought that use amorphous metal efficiently and can be implemented for high volume production of axial flux motors and the components used therein.